Synchronous generator

ABSTRACT

Slowly rotating electrical machines, for example ring generators, as are used in the wind power installations from Enercon of types E-33, E-40, E-12 and E-66, require very great excitation power. The excitation power required rises in that respect with the number of poles, with a rising air gap and with the level of the reactive power.  
     The object of the present invention is to improve the efficiency of directly driven generators of the above-described kind and to avoid the above-described disadvantages.  
     A slowly rotating synchronous generator (ring generator) for a wind power installation, wherein the generator has a rotor (rotor member) and a stator surrounding the rotor and the stator has at least one three-phase current winding on which a capacitive current is impressed and/or that a part of the exciter power of the generator is produced by the stator (FIG. 1).

[0001] Slowly rotating electrical machines, for example ring generators, as are used in the wind power installations from Enercon of types E-33, E-40, E-12 and E-66, require very great excitation power. The excitation power required rises in that respect with the number of poles, with a rising air gap and with the level of the reactive power.

[0002] Ring generators of the above-indicated kind have for example 72 or 84 poles. The efficiency of directly driven generators for use in the area of wind power should be as high as possible as they are in operation as far as possible for 24 hours a day.

[0003] Slowly operating ring generators for wind power installations such as for example those of type E-66 from Enercon operate at the rotary speed range of between 10 and 22 rpm. Such a ring generator is constructed for example with 72 poles (36 pairs of poles) and thus produces a frequency of between 6 and 13.2 Hz.

[0004] Such a ring generator comprises a rotor, through the windings of which the excitation power is built up, and a stator which surrounds the rotor. Compensation in respect of the reactive power or overcompensation of the stator with capacitors is very expensive in that respect since, as described above, the frequency is very low.

[0005] The capacitor current is generally calculated in accordance with the formula: $i_{c} = {C \cdot \frac{u}{t}}$

[0006] In that respect, for sinusoidal voltages (as in the case of known generators), there is a capacitor current of

i _(C) =U·2·π·f·C

[0007] The capacitor current is therefore determined by the voltage, the capacitance of the capacitors and the applied frequency.

[0008] In that respect, with a generator frequency of for example between 6 and 13.2 Hz, there is unfortunately only a small capacitor current, in comparison with a conventional frequency of 50 or 60 Hz. That sinusoidal capacitor current admittedly involves a phase shift of 90° relative to the active current, but it flows in each case over a range of 180° and in that situation causes increased copper losses in the stator winding.

[0009] DE 42 18 298 discloses a permanently excited generator system, wherein a synchronous generator has a rotating magnetic field which is regulatable by way of a voltage detector for detection of the output voltage of the permanently excited synchronous generator and a comparator for comparing the voltage detected by means of the voltage detector to a reference voltage which can be set by means of a voltage setting device.

[0010] U.S. Pat. No. 5, 773, 964 discloses a regulating system for an automobile generator.

[0011] The object of the present invention is to improve the efficiency of directly driven generators of the above-described kind and to avoid the above-described disadvantages.

[0012] In accordance with the invention, to attain the stated object, there is proposed a slowly rotating synchronous generator having the features set forth in claim 1. Advantageous developments are set forth in the appendant claims.

[0013] The invention is based on the technological approach that a part of the exciter power of the generator is not applied as hitherto only by the rotor (or the winding thereof) but also by the generator or the three-phase current winding thereof.

[0014] Preferably in this case the stator is excited with a capacitive current.

[0015] In this case the voltage induced in the stator is not sinusoidal in form but is in the nature of a trapezium (see FIG. 3). Then, with the trapezoidal voltage, the capacitive capacitor current flows only during the positive or the negative edge of the voltage in accordance with the formula: $i_{c} = {C \cdot \frac{u}{t}}$

[0016] The current pulses which occur in that situation are of a frequency of between about 100 and 180, preferably 130 Hz. That accordingly affords a current amplitude which is higher approximately by a factor of 10 than when a sinusoidal voltage is involved.

[0017] A further major advantage of the generator according to the invention is also that the capacitive current flows at the beginning of the entire half-oscillation. This means that the capacitive current 100% corresponds to an exciter current which can thus be reduced according to the rotor. In addition that current loads the stator winding only when there is still not a high load current loading the winding (FIG. 4). It is desirably provided that the generator stator is designed with (at least) two three-phase current windings which in turn each comprise a three-phase winding. In that arrangement the three-phase current windings are displaced through a phase angle of 30° (FIG. 5).

[0018] With that arrangement the next phase starts in each case after 30°, with a fresh oscillation. FIG. 6 shows that relationship over 360°.

[0019]FIG. 7 shows in relation to the time axis the capacitive exciter currents in the stator of the two three-phase current systems. It will be seen that every 30° electrical a fresh current pulse is delivered by the capacitors (see FIG. 5). This filter is so designed that it supplies the capacitive current peaks for the generator stator, in addition the required currents by overshoots (harmonics), which the rectifier requires.

[0020] The advantages of the construction according to the invention can also be shown by comparison with the generators hitherto, in which the exciter power is produced solely by the rotor. In the previous structure involving production of the exciter power by the rotor alone, there are approximately 20% induction losses. This means in accordance with the formula P=i²·R(100%+20%=1.2) losses of 1.2². In principle in previous generators it is not possible to avoid that loss component because the pole pieces cannot be at an infinite distance from each other and the loss of 20% is produced by the mutually juxtaposed pole pieces, insofar as magnetic loss goes from one pole piece directly into the other by way of the air gap between the pole pieces.

[0021] If now however exciter power is also produced by the stator, then such losses no longer occur in that produced part of the exciter power. This means also that the part of the exciter power produced by the stator contributes 100% to the power. Overall therefore the exciter power of the rotor can be reduced somewhat so that the loss component already goes down due to the stator exciter power, because of the freedom from loss thereof. Due to the reduction in exciter power from the rotor however stray inductance is also reduced so that the 20% loss component which occurred hitherto is reduced once again.

[0022]FIG. 8 shows a block circuit diagram of a wind power installation having a synchronous machine and a downstream-connected inverter.

[0023]FIG. 9 shows a block circuit of a wind power installation according to the invention in which a capacitance network is connected in a star point circuit to the windings of a simple three-phase current system.

[0024] A further advantage of the structure according to the invention can also be seen in FIG. 14. FIG. 14 shows the required exciter current in relation to the respective delivered power of the generator. The upper curve shows the energy demand without filter. The lower curve with an approximately 20% reduced exciter current shows operation with the structure according to the invention.

[0025] A reduction in the exciter current by about 20% produces an exciter power in the pole wheel or rotor, which is about 36% less. That represents a large reduction in the power loss of the rotor. In that way it is possible to increase the generator power. For, in the case of generators with a rated rotary speed of about 20 rpm, it is primarily the rotary speed and thus the dφ/dt or induction B in the air gap, that determine the structural size. In that way the rated power of previous generators, as in the case of wind power installations of type E-66 from Enercon (rated power 1.5 MW) can be increased to 1800 kW.

[0026]FIG. 1 shows a generator (synchronous machine SM) with a three-phase current system to which a rectifier is connected. Connected in the three-phase conductor system is a capacitance network comprising three capacitors in a delta circuit. The voltage U_(G) is applied across the individual conductors of the three-phase winding. With a sinusoidal conductor voltage a displaced sinusoidal current i_(C) is produced, as is shown in FIG. 2.

[0027]FIG. 3 shows the capacitor current with a trapezoidal voltage. FIG. 4 shows the configuration of the capacitor current i_(C) and the configuration of the load current i_(L) in the current-time diagram.

[0028]FIG. 5 shows the structure of a synchronous generator (ring generator) comprising at least two three-phase current systems, wherein each individual three-phase current system has three three-phase current windings. Both three-phase current systems are displaced relative to each other through about 30°. That is also shown with reference to FIGS. 10 and 11. FIG. 10 is a view in cross-section through a part of the extent of a slowly rotating synchronous generator according to the invention. In this case the rotor rotates within the stator.

[0029] In addition—see also FIG. 11—there are two independent three-phase current windings U₁, V₁, W₁ and U₂, V₂, W₂ in the stator. The power of the generator is thus distributed to both three-phase current windings (three-phase current systems) so that each three-phase current system has to take over only 50% of the rated power. Both three-phase current systems are displaced through an electrical angle of 30° and are thus electrically and mechanically (spatially) isolated from each other. This means that the reactance X_(D) is also approximately doubled and thus the short-circuit current is halved. This has the advantage that, in the event of a possible short-circuit in a three-phase current system, only half the short-circuit power can occur. This permits a reduction in the maximum short-circuit moment (short-circuit of two phases, for example between U₁ and V₁) by 50% in relation to a system arrangement in accordance with the state of the art.

[0030]FIG. 11 is a simple overview of the arrangement of the individual phases of the different three-phase current systems over a larger region of the stator.

[0031]FIG. 12 shows the magnetic flux in the generator according to the invention (rotor→stator). In this arrangement the magnetic flux goes directly from the pole head of the rotor to the stator uniformly between the groves.

[0032]FIG. 13 shows a view in cross-section of a wind power installation pod with a synchronous generator according to the invention. In this case the rotor of the generator is flange-mounted to the rotor of the wind power installation and the generator rotor and the wind power installation rotor are supported on a trunnion. The wind power installation rotor is accordingly connected without a transmission and without a shaft directly to the rotor of the synchronous generator. The generator rotor is disposed within the generator stator which is flange-mounted directly to the trunnion. The trunnion, like the entire drive train mounted thereon, besides the generator, is held by a machine support.

[0033] Due to the design configuration of two three-phase current windings of the generator, there are means which always limit the short-circuit torque which occurs in the event of a short-circuit at a stator winding, to a maximum of four times the rated torque, preferably twice the rated torque. The short-circuit torque can also always be less than double the rated torque. It is also possible for the generator rotor to be designed without a damping cage or a damping winding.

[0034] It will be appreciated that it is also possible to apply the arrangement according to the invention in relation to permanently excited generators. 

1. A slowly rotating synchronous generator (ring generator) for a wind power installation, wherein the generator has a rotor (rotor member) and a stator surrounding the rotor and the stator has at least one three-phase current winding on which a capacitive current is impressed and/or that a part of the exciter power of the generator is produced by the stator.
 2. A synchronous generator according to claim 1 characterised in that the synchronous generator is a multi-phase generator and that means for reducing the exciter current are provided between the individual phase conductors of the stator winding.
 3. A synchronous generator according to claim 2 characterised in that the means for reducing the exciter current are provided by capacitors and/or filter circuits for providing a stator current.
 4. A synchronous generator according to one of the preceding claims characterised in that the voltages induced in the stator are of a substantially trapezoidal configuration in the voltage-time diagram.
 5. A synchronous generator according to one of the preceding claims characterised in that the stator winding comprises at least two three-phase current systems (three-phase windings) which are respectively displaced with respect to each other through 30°.
 6. A compensation unit comprising capacitors, inductors, damping resistors, which is connected to the connecting terminals of the generator and supplies the stator with current pulses which represent an exciter current and provide the harmonic power for the rectifier which is connected downstream of the generator.
 7. A compensation unit according to claim 6 characterised in that the compensation unit supplies a sixth current harmonic for the stator.
 8. A synchronous generator according to one of the preceding claims characterised in that the generator is a permanently excited generator. 